Is it possible to create a Tenant object without Add-in Only authentication?

Hi,

Definetly possible. You don't need to create a access token if you don't want to use add-in authentication. You can use Windows authentication instead and authenticate with current Windows account or stored credentials. Just create an instance of ClientContext and set Credentials like this:

using (var actx = new ClientContext(tenantAdminUri.ToString()))
{
    //use current user
    actx.Credentials = CredentialCache.DefaultNetworkCredentials;

    //use stored credentials
    actx.Credentials = OfficeDevPnP.Core.Utilities.CredentialManager.GetCredential("MyCredentials")
}